# Copyright (c) 2014 David Lazar <lazard@mit.edu>
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

# taken from https://github.com/davidlazar/python-drbg

import hashlib
import hmac

class DRBG(object):
    def __init__(self, seed):
        self.key = b'\x00' * 64
        self.val = b'\x01' * 64
        self.reseed(seed)

    def hmac(self, key, val):
        return hmac.new(key, val, hashlib.sha512).digest()

    def reseed(self, data=b''):
        self.key = self.hmac(self.key, self.val + b'\x00' + data)
        self.val = self.hmac(self.key, self.val)

        if data:
            self.key = self.hmac(self.key, self.val + b'\x01' + data)
            self.val = self.hmac(self.key, self.val)

    def generate(self, n):
        xs = b''
        while len(xs) < n:
            self.val = self.hmac(self.key, self.val)
            xs += self.val

        self.reseed()

        return xs[:n]